With improvements in image quality and recording speed, ink jet recording apparatuses are increasingly being used in business. Examples of performance requirements required for ink jet inks for use in business include, but are not limited to, ink reliability (such as ejection stability), image quality (such as high optical density), and image fastness (such as scratch resistance and highlighter resistance). In order to improve these performance requirements, inks containing a pigment and an intramolecular-crosslinked polyurethane polymer have been investigated (Patent Literature 1 and Patent Literature 2). Patent Literature 1 discloses an ink jet ink that contains a coloring material and a cross-linked polyurethane polymer dispersion. Patent Literature 2 discloses a pigment dispersion that contains a compound having three or more hydroxy groups and a polyurethane polymer containing a soft segment having a carboxy group. The soft segment is composed of a polyol and other components.
The present inventors found that conventional inks containing a cross-linked polyurethane polymer have improved ink reliability, image quality, and image fastness but still do not satisfy recent requirements in business.
In an ink described in Patent Literature 1, the ratio of a cross-linked polyurethane polymer to a coloring material is approximately 1.0. Thus, an increase in coloring material content so as to increase optical density entails an increase in cross-linked polyurethane polymer content. This results in lower ink ejection stability. On the other hand, a decrease in cross-linked polyurethane polymer content so as to improve ejection stability entails a decrease in coloring material content, resulting in low optical density. Furthermore, the cross-linked polyurethane polymer has an acid value of 50 mgKOH/g or less and is therefore less hydrophilic. This often results in poor ink ejection stability. Thus, it is difficult to satisfy both optical density and ink ejection stability.
The present inventors found that a polyurethane polymer, as disclosed in Patent Literature 2, containing 10% by mole or more compound having three or more hydroxy groups based on all compounds having a hydroxy group becomes excessively rigid, resulting in low image fastness.